Devices for transmitting torque from one rotating shaft to another are known in the art. Such devices utilise so-called Hooke universal joints or Rzeppa constant-velocity joints to join the rotatable shafts and find application in hand tools such as socket wrenches, shown for example in U.S. Pat. No. 6,390,927, U.S. Pat. No. 6,267,681 and U.S. Pat. No. 5,458,028. In such devices the coupling joint transfers torque between the shafts, although the performance of both constant velocity and universal joints can suffer if such devices were motorised for constant torque transmission, for example, instead of being merely hand-operated. Under such load conditions, severe strain can be placed on the joint and in particular on any joining pins. In the apparatus disclosed in these prior art documents, the spring portion of the joint is present for stiffening the joint assembly and for maintaining the shaft portions at a predetermined angle to each other.
As a further example of this, U.S. Pat. No. 3,122,901 describes a universal joint assembly which delivers power from a tractor to a farm implement. Multiple joints are arranged in series at predetermined angles and a non-rotatable external spring acts as a positioning element to restrain axial movement of the joints in order to maintain the relative axial position of each joint in the series. Torque is transmitted via the universal joints but only over a limited range of relative operating angles, as controlled by the spring, in order to avoid joint damage.
Some known devices for transmitting torque over a joint involve complex linkage arrangements which can be prohibitively costly, for example the use of back-to-back universal joints linked together and located in a housing such as shown in EP407630 in an inboard-outboard boat drive train. In U.S. Pat. No. 4,065,941 in a high impact wrench, torque is transmitted through a complicated ball joint arrangement. In this apparatus, the external spring portion is used for providing some flexibility and for returning the shafts into axial alignment after each use.
In U.S. Pat. No. 1,485,036 a joint is shown which includes an external spring which transmits the rotational torque of one rotating shaft to a second shaft. The proximate ends of each shaft adjoin a ball which functions to allow the shafts to become angularly displaced with respect to one another. This apparatus is suitable in situations where a positive thrust is applied from a first shaft to the ball and then to the second shaft. However, if one shaft should suddenly have a reverse thrust applied to it, for example when the motor drive attached to the first shaft reverses (eg. when a boat drive or tractor drive is reversed), or even if the spring should become stretched over time with use, the spring can stretch open sufficiently and the ball can become dislodged from its tensioned position between the two shafts during use, and may not necessarily be received back into that position, causing failure of the joint.
AU 2002300495 discloses a joint, particularly for use in a boat drive, for transmitting torque and thrust, including a helical spring for transmitting torque and a ball and socket joint for transmitting thrust.